1. Field of the Invention
This invention relates to an evaporative emission control system for internal combustion engines, and more particularly to an evaporative emission control system of this kind, which has an evaporative fuel passage extending between the fuel tank and the intake system of the engine, and functions to prevent evaporative fuel generated in the fuel tank from emitting into the air when the filler cap of the fuel tank is removed for refueling.
2. Prior Art
Conventional evaporative emission control systems for internal combustion engines for vehicles are generally constructed such that to prevent evaporative fuel generated in the fuel tank from emitting into the air, the fuel tank is connected via a canister of the system to the intake pipe of the engine so that evaporative fuel generated in the fuel tank is adsorbed by the canister during stoppage of the engine and desorbed by the canister to be supplied to the engine for combustion during operation of the engine.
According to the conventional systems, when load on the engine is so large that the temperature of fuel in the fuel tank rises, followed by the engine being stopped, such as when the vehicle is running on an ascending slope, when the outside air temperature largely rises during stoppage of the engine, or when the vehicle is parked over a long time period, the amount of evaporative fuel generated in the fuel tank increases to an amount which cannot be adsorbed by the canister alone, resulting in emission of part of the evaporative fuel into the air. Further, when the pressure within the fuel tank is higher than atmospheric pressure due to an increase in the amount of evaporative fuel in the fuel tank, if the filler cap of the fuel tank is removed for refueling, part of the evaporative fuel in the fuel tank is emitted into the air.
To prevent such undesirable emission of evaporative fuel, it has been proposed, for example, from Japanese Laid-Open Patent Publication (Kokai) No. 60-199727, to provide an on-off valve arranged across an evaporative fuel passage extending between the fuel tank and the canister, for opening and closing the evaporative fuel passage, and a cover pivotably mounted on the vehicle chassis in a fashion covering the filler cap mounted on an oil inlet of the fuel tank, the on-off valve being interlocked with the cover such that when the cover is opened such as at refueling, the on-off valve is opened, to thereby allow evaporative fuel to be supplied from the fuel tank to the canister even during refueling, so as to reduce the amount of evaporative fuel emitted into the air from the fuel tank via the oil inlet.
According to the prior art, however, the canister cannot always be held under negative pressure during stoppage of the engine, such as at refueling, and accordingly evaporative fuel in the fuel tank is not drawn into the canister, resulting in that evaporative fuel in the fuel tank cannot be adsorbed by the canister. One way to cope with this inconvenience would be to increase the size of the canister and hence the capacity thereof for adsorbing evaporative fuel during operation of the engine. However, the increased size of the canister can unfavorably necessitate increasing a space required for accommodation of the canister.